1. Field of the Invention
The present invention relates to a grinding apparatus for reducing a thickness of a semiconductor wafer (hereinafter simply referred to as “wafer”), which is intended to be divided into plural semiconductor chips, by grinding a rear surface of the wafer. In particular, the present invention relates to a technique in which the wafer is subjected to grinding so as to have a thick reinforcing rib at an outer circumferential edge of the wafer.
2. Description of the Related Art
A Wafer having devices (for example, Integrated Circuits (=ICs) and Large Scale Integrations (=LSIs)) formed on a surface thereof is required to be made as thin as possible in order to improve it in lightness, thinness, and compactness. Therefore, in conventional techniques, the wafer has been thinned by grinding a rear surface thereof. A metal film may be formed on the rear surface of the thinned wafer by vacuum deposition or the like. However, since the wafer has a thickness of about 50 μm which is very thin, the wafer easily breaks in handling it. Thus, as disclosed in Japanese Unexamined Patent Application Publication No. 2004-281551 (in particular, in Abstract) and Japanese Unexamined Patent Application Publication No. 2005-123425 (in particular, in Abstract), a technique has been proposed in which strength is provided to the wafer by forming a thick rib on an outer circumferential edge of the wafer in grinding it for thinning the wafer. The wafer having the thick rib on the outer circumferential edge is simply referred to as “a drum-shaped wafer”.
In order to prevent breakage of active device region of the drum-shaped wafer and chips which are obtained by dividing the wafer, a technique for prevention of damage to the wafer due to processing is necessary. Therefore, when a rear surface of the wafer corresponding to an active device region formed on a surface thereof is ground, it is necessary to use a grinding stone having a grinding grain diameter which is as small as possible. However, when a wafer is ground from an initial thickness thereof by a grinding stone having fine abrasive grains, grinding is time-consuming, so that productivity is deteriorated. In addition, the grinding stone wears quickly, so that consumable tool cost increases.